This application relates to the continuous production of aromatic carbonates by reaction of dialkyl carbonates and an aromatic alcohol in the presence of a catalyst.
Aromatic carbonates, such as diphenyl carbonate, are an important reactant in the production of polycarbonate resins. Polycarbonate resins are being used in an ever increasing number of applications. Therefore, the efficient production of diaryl carbonates has become more significant. Early processes for the production of diaryl carbonates used phosgene as a reagent. However, the toxicity of phosgene prompted the development of a non-phosgene process. As shown in FIG. 1, this non-phosgene process has two-steps. First, a dialkyl carbonate, such as dimethyl carbonate (DMC), reacts with an aromatic alcohol, such as phenol, to produce an alkyl aryl carbonate (e.g., phenyl methyl carbonate, PMC) and an alkyl alcohol (e.g., methanol). Next, two molecules of the alkyl aryl carbonate undergo a transesterification reaction to produce one molecule of diaryl carbonate (e.g., diphenyl carbonate, DPC) and one molecule of dialkyl carbonate (e.g., DMC).
Various methods and apparatus for making diaryl carbonates without using phosgene are known in the art. For example, U.S. Pat. No. 5,210,268, which is incorporated herein by reference, relates to a process for continuously producing aromatic carbonates. The process is carried out in a distillation column, wherein products are recovered from the bottom of the column, and low boiling by-products are removed from the top of the column. Other processes for production of diaryl carbonates using a series of distillation columns are disclosed in U.S. Pat. Nos. 5,344,954 and 5,705,673.
U.S. Pat. Nos. 5,706,673; 5,344,954; 5,334,742; 4,182,726, and 5,380,908 describe processes for making diaryl carbonates using apparatus which comprises at least two distillation columns: the first to produce phenyl methyl carbonate, and the second to convert the phenyl methyl carbonate into diphenyl carbonate. No commercially viable apparatus has been disclosed which is capable of producing sufficient yields of diphenyl carbonate in the first column to eliminate the necessity of a second column. A single column design would make the production process more economical. Accordingly, it would be most desirable to find a process wherein the yield of PMC and DPC versus the initial phenol feed is 50% or more, and the amount of DPC produced is maximized versus the total yield of PMC and DPC. Excess production of undesirable by-products such as phenyl methyl ether (i.e., anisole) should also be avoided.
It was discovered the above goals may all be accomplished by the present invention. Specifically, it is possible to obtain a 51% yield of PMC plus DPC with a selectivity to anisole byproduct of less than 0.2%, wherein the selectivity of DPC relative to the sum of PMC and DPC was 30 to 40%. The present invention therefore provides a method for continuous production of diphenyl carbonate which has a high production rate while at the same time providing an energy and raw material efficient process.
The present invention provides a method for making aromatic carbonates. In this method, an aryl alcohol is reacted with a dialkyl carbonate in a reactor (e.g., a distillation column) to produce a arylalkyl carbonate and diaryl carbonate. The total yield of arylalkyl carbonate and dialkyl carbonate together is at least 40%. Also, the selectivity of diaryl carbonate versus diaryl carbonate and arylalkyl carbonate together is preferably at least 25%.
In the method according to the present invention, the temperature measured at the bottom of the distillation column is preferably between 220 and 240xc2x0 C., the DMC to phenol feed ratio is preferably between 4 and 7, the operating pressure measured at the top of the column is between 3 and 6 kg/cm2Gauge, and the amount of catalyst used is preferably from 0.5 to 1 molar percent.
In a more specific embodiment, the present invention provides a method for making aromatic carbonates in a distillation column having a lower reactive section and an upper rectification section. In this embodiment, a first reactive stream comprising an alcohol, and optionally a dialkyl carbonate and a catalyst, are fed into the top of the reactive section. A second stream containing a dialkyl carbonate, and optionally an aryl alcohol are fed into the bottom of the reactive section. The two streams are fed in sufficient quantities such that the weight ratio between the dialkyl carbonate and the aryl alcohol is from 4 to 6. The temperature measured at the bottom of the column is between 220xc2x0 C. and 240xc2x0 C., and the operating pressure measured at the top of the column is from 3 to 6 kg/cm2Gauge.